It has come to be recognized that substantial economies can be achieved in the repair and reconstruction of asphalt pavements if crushed salvaged asphalt material, reclaimed from a previously laid pavement, is mixed with virgin (unused) aggregate. Heretofore, however, there has been no fully satisfactory apparatus for accomplishing continuous hot mixing of virgin and salvaged asphalt materials.
Conventionally, apparatus for continuous mixing of virgin asphalt material comprises an elongated drum rotatable about an axis that is substantially horizontal but is tilted downward towards a front discharge end of the drum. Aggregate material is charged into the rear end of the drum and moves forward by gravity as the drum rotates, while flights or lifts on the inner surface of the cylindrical side wall of the drum agitate and tumble the material. To heat and dry the aggregate as it enters the drum and moves forward in it, a fuel burner projects a flame into the rear end portion of the drum, substantially along the drum axis. Liquefied asphalt cement is introduced into the drum some distance forward of the burner flame, as by means of a duct that extends into the drum from one of its ends. The asphalt cement must be brought into contact with the aggregate when the latter has been highly heated, but it must not enter the drum near the burner flame lest it be heated to a temperature at which it smokes. Smoking should be avoided not only because the binding properties of the asphalt are degraded by the chemical breakdown denoted by smoke but also because the smoke from overheated asphalt is extremely dense and noxious.
Obviously, salvaged asphalt material cannot be introduced into the rear end of a conventional mixing drum, along with the virgin aggregate, because the heat of the burner flame would cause smoking of the asphaltic binder in the reclaimed material. Various arrangements have therefore been proposed for introducing salvaged asphalt material into a mixing drum in such a manner as to prevent its being heated to smoking temperature, but heretofore no completely successful apparatus has been devised by which salvaged asphalt material can be mixed with virgin materials. To be fully satisfactory, such apparatus should be inexpensive, should provide for continuous throughput as opposed to batch processing, should have a high production rate during both recycling operation with salvaged asphalt material and standard operation with all virgin material, should be very quickly and easily convertible from either type of operation to the other, should require little maintenance, and should of course be smokeless.
One prior arrangement intended for use with salvaged asphalt material is represented by several U.S. Patents to R. L. Mendenhall, including No. 3,845,941; No. 4,067,552 and No. 4,074,894. These patents disclosed a mixing drum through which heating tubes extended lengthwise, and hot combustion gases were passed through the tubes for indirect heating of materials in the drum. This apparatus was substantially smokeless, but it was very expensive and had a low rate of production for its size and cost so that it was not satisfactory for standard operation with virgin materials.
Another prior arrangement for mixing salvaged asphalt material consisted of a short, small diameter drum that was coaxially telescoped partway into the rear inlet end portion of a longer, larger diameter drum. A burner flame was projected coaxially into the rear of the smaller drum and virgin aggregate was charged into the rear of that drum to be directly heated by the flame. Salvaged aggregate material was charged into the rear end portion of the larger drum, to be indirectly heated by the flame and to be mixed, while moving forward in the large drum, with the heated virgin material issuing from the front of the small drum. The small drum was removed when all-virgin materials were to be mixed, and the apparatus then had a substantially higher production rate, which is to say that it had a low rate of production when operating with salvaged materials. Owing to the need for installation or removal of the small drum, the apparatus was not readily convertible from one type of operation to the other.
U.S. Pat. No. 4,075,710, to Jakob et al, discloses apparatus which was essentially a conventional drum mixer for virgin materials, modified by the addition of a conveyor that projected rearwardly into the drum from its front discharge end and terminated a distance forwardly of the fuel burner flame, for feeding salvaged asphalt material into a medial portion of the drum. The apparatus was expensive in itself and was also expensive to maintain because moving parts of the conveyor were located in a highly heated zone. The conveyor had to be removed for standard production of all-virgin mix, to minimize conveyor maintenance and because the apparatus had a low production rate in its configuration for mixing salvaged material.
In the drum mixer of U.S. Pat. No. 4,039,171, to Jakob et al, a baffle or heat dispersing grid was mounted in the drum ahead of the burner flame, with the intention of preventing the highly heated burner gases from coming into direct contact with salvaged asphalt material fed into the drum along with virgin aggregate material. However, the apparatus reportedly produced an objectionable amount of smoke when operating with salvaged material. Furthermore, the heat dispersing grid had to be made of heat resistant alloy and was therefore expensive, and it had to be removed for satisfactory use of the mixer with all-virgin materials because the apparatus had a low production rate with the grid in place.
U.S. Pat. No. 3,999,743, to R. L. Mendenhall, discloses a mixer having circumferentially spaced ports in its drum, arranged in one or more zones intermediate the ends of the drum, through which salvaged asphalt material could enter the drum at some distance forward of the burner flame. At each port a scoop was fixed to the side wall of the drum and projected into an annular trough that embraced the drum. Salvaged material was filled into the trough through a chute at its top. As the drum rotated, the several scoops picked up material from the trough and charged it into the interior of the drum through their respective ports. This arrangement was incapable of feeding salvaged asphalt mix into the drum at any substantially high rate, and accordingly it did not lend itself to high rates of production. Furthermore, material moving forwardly through the zone of the ports could fall out of the drum through them at any time material was not being forced into the drum by the action of the scoops, and therefore the apparatus was not suitable for operation with virgin aggregate materials unless the ports were closed in some manner for such operation.
Still another arrangement is disclosed in U.S. Pat. No. 4,147,436 to David L. Garbelman wherein the recycled material is fed into the rotating drum by being dropped into it to the bottom thereby directly passing through the hot flame. To preclude direct exposure of the recyclable aggregate to the hot flame there is provided a flame deflector or heat shield. Also, to prevent the aggregate introduced into the drum at the top so that it remains in the drum until discharged, there is provided a plurality of feeding port covers which close under the influence of gravity as the drum rotates.
Other arrangements have been proposed for feeding material into a rotatable drum through ports or the like in a medial zone thereof, wherein hinged doors or flop gates on the drum opened to permit introduction of material into an upper portion of the drum and closed to prevent escape of material from a bed in the lower portion of the drum. Such hingedly mounted elements were noisy, tended to accumulate build-ups of asphaltic material that interfered with their proper functioning, and had to be secured against opening for use with all-virgin materials and had to be released for conversion to use with salvaged materials.
By contrast with the prior state of the art, the general object of the present invention is to provide a rotatable drum mixing apparatus that provides for charging of materials into the mixing drum in a zone thereof that is between its ends, which apparatus has no parts that are movably mounted on the drum and is therefore highly reliable and easy to maintain, provides for a high and steady rate of feed of material into said medial zone of the drum, prevents escape of materials from the interior of the drum while facilitating passage of materials through said zone, and permits the apparatus to be converted almost instantaneously from its salvaged asphalt material mixing mode to an all-virgin materials mode.
Another general object of the invention is to provide mixing apparatus of the type comprising a drum that is rotatable on a substantially horizontal axis and has an inlet for one kind of material at a rear end thereof and an outlet for discharge of mixed material at its forward end, which apparatus has means for feeding another kind of material into a zone between the ends of the drum at a high and steady rate but is nevertheless so arranged that material moving through said zone from the rear end of the drum cannot escape from the drum in said zone.
A more specific object of the invention is to provide a rotary drum mixer that is suitable for use with either salvaged asphalt material or all-virgin material, with high rates of production in both cases, and which can be converted for use with one or the other of such materials by the mere stopping of the recycle feed conveyor.
It is a further specific object of this invention to provide a rotary drum mixer wherein the drum has circumferentially spaced charging ports around a zone between its ends, and wherein means which are fixed on the drum to rotate with it, and which do not move relative to it, guide infeed material through the ports to the interior of the drum from a hopper above it and also prevent the incursion of external air into the drum and the escape of solid materials that move through the interior of the drum past the zone of the ports.
It is also a specific object of the invention to provide a rotary drum mixer that has a ported cylindrical side wall, wherein said side wall is reinforced at its ported zone by chute structure which serves to guide material into the drum from a hopper above it.